1. Field of the Invention
This invention generally is an apparatus and method for ionospheric mapping and more specifically the obtaining of a detailed spatial and temporal ionospheric mapping utilizing information received from global positioning system (GPS) satellites by ground-based GPS receivers.
2. Description of Related Art
Over the past several years, with the availability of a full constellation of global positioning system (GPS) satellites, the use of calibrated GPS receivers for global ionospheric measurements has become feasible. The theory of the GPS is based on measuring the travel time, .DELTA.t, of a signal from the satellite to a receiver.
Measurement of the integrated ionospheric path delay can be accomplished because the index of refraction in the ionosphere has sufficient dispersion at L-band frequencies. To deal with ionospheric path delay, the GPS provides travel time, i.e., ranging data at two frequencies, by cross-correlation between the P-code (precise code) signals of the two frequencies without demodulating the P-code signal. These are the L1 and L2 frequencies: 1575.42 MHz and 1227.6 MHz. One of the major difficulties encountered has been the determination of the L1-L2 spacecraft biases for each of the GPS satellites.
A comparison of preflight calibration and analysis of data from the GPS satellites show that there is an instrument delay time bias between L.sub.1 and L.sub.2 frequencies which differs for each satellite. A constant L1-L2 bias would result in a constant error in the ionospheric path delay determination and range measurements.
A method of determining the SV bias has been the measurement of the ionospheric total electron content (TEC) over several simultaneous propagation paths. This TEC is a function of many variables including long and short term changes in solar ionizing flux, magnetic activity, season, time of day, user location and viewing direction. Under typical conditions, continuous slant path TEC data can be measured simultaneously from several GPS satellites over a period of 90-120 minutes. One of the important factors limiting the measurement of ionospheric TEC using calibrated GPS receivers is the uncertainty in the measurements caused by differential L2-L1 instrumental delay biases in both the satellite transmitters and the receivers. The receiver bias errors require that the receiver be returned to the manufacturer on a periodic basis for recalibration, a procedure that is expensive and results in the loss of an expensive piece of equipment for an extended period of time. Another method requires use of expensive GPS simulator equipment.
Several researchers have proposed methods for determining the space craft biases and ionospheric time-delay and therefore a usable mapping of the ionosphere have been attempted. However, these attempts have resulted in techniques that have high error rates or require large investments in equipment spread over the world. The most accurate system of ionospheric mapping in use today was developed by the Jet Propulsion Laboratory (JPL) of the California Institute of Technology in Pasadena, Calif. This system consists of a global network of tens of receivers for use in calibrating the JPL Deep Space Network (DSN).